“It was a moment without the museum being a bank. No guards, no ropes, no limits, no pedestals – a free moment of improvisation. Today all that energy and participation has been cancelled. Only things that can go into an auction house become art.”

Germano Celant, the eminent 73-year-old Italian curator, is talking to me about When Attitudes Become Form, Harald Szeeman’s seminal show at the Kunsthalle Bern, Switzerland, which introduced post-minimalist strategies of process art, conceptualism, land art and performance into a museum setting.

It was 1969 and Richard Serra splashed 210kg of lead in the foyer, Alighiero Boetti placed concrete pieces on the floor to depict “Me Sunbathing in Turin”, Joseph Beuys installed a “Fat Corner”, Lawrence Weiner took a chunk out of the wall, Bruce Nauman laid out “Collection of Various Flexible Materials Separated by Layers of Grease with Holes the Size of my Waist and Wrists”, Alain Jacquet exposed the Kunsthalle’s electrical wires and left them hanging, and “Art by Telephone” consisted of a telephone and a sign reading “When the phone rings, pick up the receiver. Walter de Maria will be on the phone wanting to talk to you”.

The show cost Szeeman his job in Bern, but brought international renown. Celant, who gave the opening speech in 1969, is now engaged in one of the most obsessive, ambitious projects of his career: to reconstruct the historic exhibition, grafting it in its entirety – modernist white walls, floors, installations and art objects – on to the splendid baroque frescoed decor of the Prada Foundation’s Ca’ Corner della Regina in Venice.

“The ‘impossible’ encounter of the 1918 Swiss building with the 18th-century Venetian palazzo is an attempt to introduce the relationship between things into art history. I think this approach got lost in the recent past due to the shattering caused by the commerce that built up related to collecting,” Celant says.

The exhibition launches during the Venice Biennale and its audience will come from precisely that commercial milieu – the collectors, dealers, curators and advisers currently on their annual spring pilgrimage, moving en bloc from Frieze New York to art fairs in Hong Kong and Basel via Venice. There are rich paradoxes here. The structured chaos of When Attitudes Become Form stands as a giant reproach to the smooth packaging of today’s art market – the very market that has tamed the post-minimalist positions of Nauman, Serra et al to make them star turns at fairs and museums.

“The material was from the street, everyone was working in the same room, there were no separate territories. You are in a situation, you play with it, instead of bringing in this product from the studio. Today that would not be possible because of the economic value of the work,” Celant reminisces. “All the concentration was about breaking the role of the museum. Now, in Venice, the modern is the past and the baroque is the present.”

The works’ fragile character, plus the contradiction that many of Bern’s subversions became museum trophies – Nauman’s “Various Flexible Materials” is among several pieces now in the collection of New York’s Museum of Modern Art – have made loans difficult, though Celant has replacements where necessary, and artists are collaborating on reconstructions.

“The biggest doubt was how to make this a contemporary rather than a nostalgic event,” says Celant. Yet to me the show’s 21st-century resonance shrieks. It is not only that it marks the dawn of a sort of creativity – the breakdown between art media, the emphasis on process-based work – which continues to be enormously influential, and to need curatorial explanations. It is also that Venice – especially this year as it comes hot on the heels of Frieze New York and Art Basel Hong Kong – dramatises an extreme tension in current art: between the commodified and spectacular (François Pinault’s Palazzo Grassi, for example), and the downbeat ideological biennale-speak about migration, displacement, anti-commodity, which is something like its mirror image. The coexistence of these two approaches takes us back to 1960s idealism, when art and politics were not separated like this, and to Szeeman’s radical concept of an exhibition as “not just a group show but a temporary world”.

At the start of Celant’s project, Rem Koolhaas, its architect, reckoned it “an irony that an exhibition that was so much about improvisation and artistic freedom would be framed in such classical art-historical terms. In the end, the enforced marriage of the two architectures in the Venetian building was a relief – that there would be an experimental dimension ... I have always been very interested in archaeology, [where] you love to untangle very complex architectural aggregates and layers that all occupy the same space. This project felt like an archaeological project to be untangled by the viewer.”